Recently, in a basic DWDM (Dense Wavelength Division Multiplex) optical communication system, a relaying optical amplifier is used on an optical transmission channel. For constraint of transient response characteristics of the optical amplifier, generally, it is a requirement for an optical transmitter which becomes an optical input source to the optical amplifier to have a specification which takes a predetermined time period at a startup and then gradually increase an optical output.
FIG. 1 is one example of the specification at the startup of the optical output required for the optical transmitter. As illustrated in FIG. 1 (1), when a setting signal of an optical source of the optical transmitter changes from OFF to ON, as illustrated in FIG. 1 (2), with the optical output of not less than Pmin, the optical output is slowly started with an inclination within the specification range (0 to 1 mW/sec). Here, in the case of the optical output of not greater than Pmin, since the optical amplifier on the transmission channel does not react, the control is not required. When the optical output becomes greater than Pmin, since the optical amplifier starts to respond, the control is performed so as to gradually increase the optical output.
FIG. 2 explains a content of the control performed when the optical output gradually increases.
Conventionally, the inclination at the startup of the optical output used to be moderated by controlling injected current to a laser diode (LD) of the optical transmitter and by increasing the LD optical output to a prescribed value, for example, linearly, taking a predetermined time period. In addition, when the optical transmitter consists of the LD and a phase modulator, it is a requirement that a bias voltage or an amplitude of an electric signal which drives the phase modulator is controlled to have an optimal value by Auto Bias Control (ABC control). The control is started when an optical input (LD optical output) to the phase modulator reaches Pmin. In the ABC control, the control may be performed by superimposing a pilot frequency which is a low frequency on an optical signal and by detecting the components.
When the phase modulator consists of a plurality of stages, generally, the ABC control of each stage is performed by a time division. Here, explanation is given for the ABC control of a DP-QPSK (Dual Polarization Quadrature Phase Shift Keying) modulation scheme. In performing control by a time division, each time section is divided into I to VI, and the control of the sections I to VI is repeated until the optical output of the LD reaches the prescribed value.
FIG. 3 illustrates a configuration of a DP-QPSK modulator. The modulator of FIG. 3 consists of a MUX 10 which sends out signals to each of driving circuits 12-1 to 12-4, an LD 11 which generates DC light, and a DP-QPSK LN modulator 13. The DP-QPSK LN modulator 13 consists of an X side QPSK modulator 15 which generates optical signals of a horizontally polarized wave and a Y side QPSK modulator 14 which generates optical signals of a vertically polarized wave.
Driving circuits 12-1 to 12-4 apply driving signals which correspond to data signals to modulating electrodes 16-1 to 16-4 of the X side QPSK modulator 15 and of the Y side QPSK modulator 14, respectively. In the X side QPSK modulator 15 and the Y side QPSK modulator 14, an arm which generates I signals and Q signals is provided, respectively, and as the modulating electrodes 16-1 and 16-2, the electrodes for the I signals are provided, and as the modulating electrodes 16-3 and 16-4, the electrodes for the Q signals are provided, respectively.
Bias controlling units 17-1 to 17-4 are provided in each of the arm for the I signals and the arm for the Q signals of the X side QPSK modulator (phase modulator) 15 and the Y side QPSK modulator (phase modulator) 14. The bias controlling units 17-1 to 17-4 control the bias of a modulating operation in each arm. π/2 Shift bias controlling units 18-1 and 18-2 control the bias so that the optical phase difference of the I signals and the Q signals generated by the X side QPSK modulator 15 and the Y side QPSK modulator 14 become π/2 accurately. Monitoring PDs (Photo Diodes) 19-1 and 19-2 detect light after the modulation of the X side QPSK modulator 15 and the Y side QPSK modulator 14 for each control. A 90° polarization rotator 20 rotates a polarized wave of the optical signal from the X side QPSK modulator 15 by 90° with respect to the optical signal from the Y side QPSK modulator 14 and realizes polarization multiplication.
Hereinafter, explanation is given for the time division ABC control by referring to FIGS. 2 and 3. FIG. 2 illustrates an example in which 12 second-time sections are divided into six and performs the ABC controls which are different in each section.
In section I, bias control is performed using a π/2 Shift bias controlling unit 18-2 of the X side QPSK modulator 15. In section II, the control of the modulation amplitude by the driving circuit 13 for the I signals of the X side QPSK modulator 15 and the bias control of the arm of the I signals of the X side QPSK modulator 15 are performed. In section III, in the arm of the Q signals of the X side QPSK modulator 15, the control of the modulation amplitude by the driving circuit 12-4 and the bias control by the bias controlling unit 17-4 are performed. In section IV, the bias control of the π/2 Shift bias controlling unit 18-1 of the Y side QPSK modulator 14 is performed. In section V, the control of the modulation amplitude by the driving circuit 12-1 of the arm of the I signals of the Y side QPSK modulator 14 and the bias control by the bias controlling unit 17-1 are performed. And in section VI, the control of the modulation amplitude by the driving circuit 12-2 of the arm of the Q signals of the Y side QPSK modulator 14 and the bias control by the bias controlling unit 17-2 are performed. Here, as an example, each section is defined to take 2 seconds.
The monitoring PDs 19-1 and 19-2 detect the optical output of the X side QPSK modulator 15 and the Y side QPSK modulator 14 every time the controls of I to VI are performed.
In controlling sections II, III, V, and VI, the bias control and the amplitude control are performed by superimposing two different types of pilot frequencies on each optical signal and detecting it.
Concerning such time division ABC controls, please seethe Patent Documents 2 and 3.
Some in the conventional art perform optimal control of driving signal amplitudes, DC biases, and phase shifters which are related to optical phase modulators such as DQPSK modulators, QPSK modulators, and the like.    Patent Document 1: Japanese Laid-open Patent Publication 2007-208472    Patent Document 2: Japanese Laid-open Patent Publication 2008-092172    Patent Document 3: Japanese Laid-open Patent Publication 2007-082094
In the conventional art, right after the startup of the ABC control (in particular, the control of bias voltages and of amplitudes), a) optical power loss in the phase modulator greatly fluctuates by the large movement of the ABC control toward its convergence point (optimal point). In addition, b) generally, the phase modulator and its driving circuit are implemented nearly because of high frequency characteristics. Accordingly, when the output amplitude of the driving circuit increases, consumption power of the driving circuit increases, and more heat is produced, and because of the heat, a temperature rise of the phase modulator is generated. With this effect, light transmission characteristics (applied voltage vs optical power loss) of the phase modulator is shifted in a short time period, and therefore, the optical power loss in the phase modulator greatly fluctuates in the section where no bias voltage is controlled (the section where the voltage is kept).
FIGS. 4 and 5 explain phenomena in which the optical power loss in the phase modulator greatly fluctuates by a large movement of the ABC control toward its convergence point (optimal point).
FIG. 4 is a graph in which a horizontal axis illustrates an applied voltage of the phase modulator (which is a modulation voltage applied to the modulating electrode of the phase modulator and which is also called a driving voltage) and in which a longitudinal axis illustrates an optical power loss of the phase modulator (optical output of the optical transmitter). The modulation voltage or the driving voltage is controlled by the modulation amplitude in which the voltage for modulation changes, and the bias voltage which is the main voltage of the change).
The optical power loss of the phase modulator (optical output of the optical transmitter) here illustrates the change in the optical output at the time when the applied voltage of the phase modulator changes when the optical output of the LD is the largest. FIG. 4 illustrates the optical power loss characteristics of the phase modulator which has sine curve characteristics, and here, the applied voltage illustrated in the view below the horizontal axis is illustrated. The central point of the amplitude of the applied voltage is called a bias point, and this bias point may sometimes greatly change by the ABC control. In FIG. 4, the bias point is illustrated in black dots, and in the case of the phase modulator, the optimal position of the bias point is the point at which the optical output becomes the smallest.
When the applied voltage moves from (1) to (2), the optical output is illustrated in (3) when the applied voltage is (1), and the optical output is illustrated in (4) when the applied voltage is (2). As seen from comparison of the optical outputs (3) and (4), the power loss of the optical output (optical output) greatly moves.
FIG. 5 is a graph in which a horizontal axis illustrates an applied voltage of the phase modulator and in which a longitudinal axis illustrates an optical power loss of the phase modulator (optical output of the optical transmitter). FIG. 5 illustrates the case where the applied voltage is initially (1), but with the ABC control, the amplitude of the applied voltage suddenly enlarges as illustrated in (2). The optical output is (3) when the applied voltage is (1), and the optical output is (4) when the applied voltage is (2). As seen from comparison of the optical outputs (3) and (4), the optical output greatly fluctuates.
FIG. 6 explains phenomena in which the optical power loss in the phase modulator greatly fluctuates by a shift of light transmission characteristics by a temperature rise.
FIG. 6 is a graph in which a horizontal axis illustrates an applied voltage of the phase modulator and in which a longitudinal axis illustrates the optical power loss of the phase modulator (optical output of the optical transmitter). In FIG. 6, the light transmission characteristics illustrated as a sine curve are shifted to the right by a rapid temperature rise. It is understood that the optical output changes from (1) to (2) although the applied voltage is not changed.
FIG. 7 explains problems shared by the conventional techniques.
As a result of the above mentioned a) and b), such cases occur that the optical output startup inclination of the optical transmitter becomes steep or negative as illustrated in FIG. 7, which means that the optical output cannot be started with the inclination within the specified range.
FIG. 7 is a schematic view and does not strictly illustrate that the inclinations become steep or negative in sections of II, III, V, and VI as illustrated in FIG. 7. By performing the ABC control of sections I to VI, however, such cases may occur that the inclination of the optical output becomes steep or negative in any of these sections.
By the phenomena, a level of the optical input to the optical amplifier which is located as a relaying device on an optical transmission channel of the DWDM optical communication system rapidly changes, or is subject to increase and decrease (ringing). Since a response time of this optical amplifier for the gain control (AGC: Automatic Gain Control) is slow, it cannot follow the rapid change or ringing of the optical input level, and therefore, the optical output level from the optical amplifier transiently changes. This change in the level of the optical output causes deterioration of a transmission quality.
FIG. 8 explains generation of deterioration in the transmission quality in a DWDM optical communication system.
It is presupposed that optical signals with each wavelength are output from the optical transmitters 1 to N, and that a steep change in the optical output or ringing occurs from the optical transmitter 1. The optical signals with each wavelength from the optical transmitters 1 to N is wavelength-multiplexed in the MUX 30 and sent. Wavelength-multiplexed optical signals which were wavelength-multiplexed in the MUX 30 are amplified in a multiplexed state in the optical amplifier 31. Here, by the occurrence of the steep change in the optical output or ringing, the steep change or ringing also occurs over the entire wavelength-multiplexed optical signal as well. Since, however, the response time of the AGC control in the optical amplifier is slow, it cannot follow the steep change or ringing, and the optical output from the optical amplifier 31 indicates some changes which are different from the steep change or ringing. If it could follow the steep change or ringing, the optical signals with the wavelength other than the wavelength at which the ringing occurred would be kept constant, however, in reality, it cannot, and therefore, the change of the optical output occurs in the optical signal with other wavelength as well.
Therefore, when such wavelength-multiplexed optical signals are demultiplexed by a DeMUX 32 and are received by each of the optical receivers 1 to N, change occurs in the optical output with respect to the optical signals with every wavelength. This change in the optical output causes deterioration of the transmission quality in a receiving side.